Generation of VOC was required to be reduced by 30%, including voluntary controls, in 2010 as a result of having been designated as a harmful chemical substance that leads to global warming. In the field of paints, since large amounts of organic solvents are used as viscosity lowering agents used in paints, and the paint industry is the largest generation source of VOC, accounting for nearly 60% of the total of roughly 1,500,000 tons of VOC generated in Japan, the implementation of VOC countermeasures is becoming an opportune issue for the paint industry.
In the area of coating industrial technology, the main form of coating consists of spray coating using a diluent solvent, and various measures have been devised to reduce VOC levels. For example, technologies have been developed for changing over to aqueous paints, using paints, namely high-solid paints, that have reduced levels of organic solvents, or recovering and decomposing discharged organic solvents.
However, among these countermeasure technologies, the present circumstances are such that changing over to aqueous paint requires additional equipment such as a water treatment system and air-conditioning equipment, and although aqueous paint is suitable in the case the object to be coated is a metal material, it is unable to accommodate coating onto plastic components and the like requiring high coating finish quality.
Thus, in the paint industry, and particularly at small- and medium-sized firms, attempting to accommodate the above-mentioned VOC countermeasures presents problems such as requiring considerable equipment investment with the current level of technology, and as a result, there is a strong need for the development of a novel coating technology that is able to take the place of current organic solvent coating or aqueous coating.
On the other hand, with respect to coating technology, a technology has been proposed in a patent filed by the Union Carbide Corp. of the U.S. (Patent Document 1) that uses a supercritical fluid instead of an organic solvent as a viscosity reducing agent (diluent). In this technology, a supercritical fluid, and particularly carbon dioxide, is dissolved in a paint (consisting of a polymer and a true solvent that dissolves the polymer and imparts fluidity) and enables coating by lowering viscosity to a level that enables spraying.
This firm has subsequently filed more than ten patents relating to coating technology, five of which have been registered as patents, including a method for controlling spray width (Patent Document 2), limitation of paint composition (Patent Document 3), a method for improving sprayed state (Patent Document 4), a method for avoiding blockage (Patent Document 5), and a method for controlling density of a paint/CO2 mixture (Patent Document 6).
However, these patents only indicate a single process flow, while the construction of a practical process flow and specific operating methods have had problems in terms of lacking an adequate degree of completeness. Namely, spray coating technologies that use carbon dioxide as a viscosity lowering agent have essentially yet to be established from the viewpoint of being practical industrial technologies.
The process flow indicated in the above-mentioned patents is shown in FIG. 1. In this flow, paint and CO2 are compressed with a pneumatic piston pump, and the paint is heated and sent to a mixer for the purpose of lowering viscosity. The CO2 is sent directly to the mixer in a liquid state without being heated. The paint and CO2 supplied at that time are supplied at a constant volume ratio by a mechanism that simultaneously controls the movement of the pistons of both pumps.
The mixer is a fluid multi-stage separation-type static mixer, and after the mixture is heated, it is passed through a filter, is again mixed by a static mixer, and is sent to a spray gun after reducing in pressure as necessary. In the spray gun, the mixture is sprayed at a flow rate determined according to the pressure. Surplus mixture is pressurized in a circulation line after which it is returned to the line beyond the first static mixer.
Although the above describes the general process flow of carbon dioxide coating proposed in the prior art, when attempting to make this flow practical, there is presumed to be the problem of being unable to achieve a balance between the supplied amounts of paint/CO2 and spray volume, resulting in the risk of, for example, an increase in pump discharge pressure. In this patent, although a configuration is employed that causes paint and CO2 to be released into the atmosphere if pump discharge pressure becomes equal to or greater than a fixed pressure, the release thereof into the atmosphere is not preferable in terms of operation, and is also economically disadvantageous since the paint and CO2, and especially CO2, cannot be reused.
In addition, although a circulation line is provided, this is not particularly required in terms of operation. Moreover, although an electrical heating method is used to heat the paint and mixture, in the case of employing this method, since it is difficult to control the temperature to a constant temperature when starting operation of the device and when changing the flow rate, the use of another type of heating method is desired. In addition, during actual coating work, spraying is frequently momentarily interrupted or stopped for a fixed period of time such as when changing the coated surface or when changing the object to be coated per se.
In such cases, in the proposed process flow, since problems such as increases in system pressure are presumed to occur, considerable improvements are required in order to achieve a realistically feasible process flow. Moreover, there are no considerations given whatsoever to cleaning the lines at completion of coating, which is considered to be important in the case of actual coating. In this manner, the carbon dioxide coating indicated in the above-mentioned series of patents clearly does not constitute a practical process flow configuration constructed at a level that can be applied as an actual coating technology, and there is a strong desire for the construction of a process flow able to be applied practically in this technical field.
Moreover, among the above-mentioned patents, a method for avoiding blockage as indicated in Patent Document 5 is extremely important in terms of practical application of this coating technology. According to this patent, it is indicated that a dense substance for coating that contains a cellulose-based polymer such as nitrocellulose or cellulose acetate butyrate forms a solid precipitate in the mixer, and if operation is allowed to continue, causes an increase in pressure and ultimately results in blockage that prevents further spraying.
This document indicates that, although an attempt to resolve this problem was made by making changes to equipment, including an examination of fluid dynamics and a reexamination of the type of mixer, even though operation was possible for a short period of time, since long-term, stable operation was not possible, the problem of blockage was resolved not through equipment accommodations, but through procedural accommodations, namely by placing limitations on the operating temperature and pressure.
More specifically, this patent indicates that the compressed fluid used to lower viscosity is air or a supercritical fluid at the time of mixing with paint, and is manipulated at a temperature and pressure so as to have a solubility coefficient of about 0.5 cal/cc to about 4.0 cal/cc, thereby preventing deposition of the solid polymer.
On the basis of this patent, when the inventors of the present invention also carried out carbon dioxide coating of a one-liquid curing type paint (acrylic resin-based paint blended with nitrocellulose), blockage was confirmed to occur in the mixer under numerous temperature and pressure conditions. Moreover, problems in the mixture caused by blocking were confirmed to occur frequently and prevent coating from being carried out even under conditions of about 0.5 cal/cc to about 4.0 cal/cc, such as at a pressure of 8 MPa or less under constant temperature conditions of 40° C.
In this patent, this solid deposition phenomenon is discussed from the viewpoint of phase equilibrium, and solubility coefficient is limited as a condition for preventing polymer deposition. As a result of investigating and studying the circumstances involved in polymer deposition in detail with regard to this solid deposition phenomenon, the inventors of the present invention determined that polymer deposition occurs in a single carbon dioxide line immediately before mixing prior to deposition in the mixer, and that as a result thereof, the supply pressure of the carbon dioxide supply line increases rapidly.
These findings suggest that, as a result of backward flow of a portion of the paint into the carbon dioxide supply line to the mixer and the solvent component in the mixture (true solvent) being extracted and removed into carbon dioxide, namely the liquid carbon dioxide or supercritical carbon dioxide, the polymer component in the paint precipitates thereby causing a phenomenon that causes blockage.
Paint does not inherently flow back into the carbon dioxide line during normal operation. However, if pressure fluctuates (pressure increases) due to the presence of a flowing state downstream from the mixer, although pressure immediately increases since the paint is an incompressible fluid, since carbon dioxide is a compressible fluid, a time difference occurs in the pressure rise, and backflow of the paint into the carbon dioxide line is presumed to occur during that time. Namely, coating technology in which carbon dioxide is used as a viscosity lowering agent for a highly quick-drying paint in the manner of a one-liquid curing type paint is considered to have been yet to be established from the viewpoint of an industrialized technology, and in this technical field, there is a strong desire for the development of a novel technology capable of solving these problems while also being able to be applied practically.    Patent Document 1: Japanese Patent Publication No. 1927328    Patent Document 2: Japanese Patent Publication No. 2061845    Patent Document 3: Japanese Patent Publication No. 2670904    Patent Document 4: Japanese Patent Publication No. 2785099    Patent Document 5: Japanese Patent Publication No. 2739548    Patent Document 6: Japanese Patent Publication No. 2807927